1. Field of the Invention
This invention relates to an outer shell for a socket of a joint prosthesis, and in particular relates to an acetabular shell having closed off access channels within the inner aspect of the shell wherein the closed off section of each access channel can be removed for receipt of a bone screw or penetrated by a bone screw for screw fixation of the acetabular shell to bone.
2. Description of the Related Art
For many years now, prostheses have been implanted in the human body to repair or reconstruct all or part of an articulating skeletal joint, such as the hip joint. The hip joint includes the femur and the pelvis, each of which has a surface for articulation against an adjacent articulation surface of the other bone. The femur has a head having a convex, generally spherically contoured articulation surface. The pelvis includes an acetabulum having a concave, generally spherically contoured articulation surface. The articulation surfaces of the femur and the pelvis form a ball-and-socket type joint.
One or both of the articulation surfaces of the hip joint may fail to perform properly, requiring the defective natural articulation surface to be replaced with a prosthetic articulation surface. In an artificial hip joint, a femoral head and a femoral stem can be used to replace the natural head, stem, and articulating surface of the femur, and an acetabular cup can be used to replace the natural socket and articulating surface of the acetabulum of the pelvis. The artificial femoral stem and head may be an integral unitary component or separate modular components designed to be assembled together. The femoral head articulates directly against the natural acetabulum or the artificial acetabular cup. The acetabular cup component is received and fixed within the acetabulum of a pelvis. The pelvis is prepared to receive the acetabular cup by reaming a concavity in the acetabular bone. The acetabular cup component typically has an outer surface conforming to the concavity reamed in the acetabular bone of the pelvis, and an inner bearing cavity for receiving the head of the femoral component or a healthy femur. The head articulates in the bearing cavity of the acetabular cup.
One known type of acetabular cup includes an acetabular shell made of a bio-compatible metal, such as titanium or a titanium or cobalt alloy, and a bearing insert made of a material which allows the natural or artificial femur head to move about, such as a bio-compatible polymer (e.g. ultra-high molecular weight polyethylene). Some acetabular shells are attached to the acetabular bone using polymerizable synthetic cement, and others are attached to the bone using mechanical anchoring means such as screws. The shell also can be affixed by a combination of bone screws and bone cement. Still other acetabular shells can be attached to the acetabular bone using a xe2x80x9cpress-fitxe2x80x9d shell in which the shell is inserted forcibly into the acetabular cavity. After the acetabular shell is implanted, the bearing insert is secured within the acetabular shell and the head of the femoral component or a healthy femur is positioned in the bearing insert.
If bone screw affixation is selected for the acetabular shell, the bone screws are driven into the acetabular bone through screw holes in the acetabular shell. Often, the acetabular shell is provided with more screw holes than typically would be used by the implanting physician. This provides a selection of sites for placement of the bone screws, as may be dictated by the condition of the patient""s pelvic bone. Some of the provided screw holes may receive a screw while others do not. When the assembled hip prosthesis is loaded, unintended micromotion may occur between the acetabular shell and the bearing insert. This micromotion may form wear debris (e.g., fine polyethylene or metal particles) from the rubbing of the acetabular shell and the bearing insert. It has been reported that this wear debris may blend with synovial fluid and thereby migrate out of the screw holes of the acetabular shell and into the acetabular bone region. The wear debris may cause osteolysis, which can lead to bone resorption and loosening of the bone screws over time.
In order to limit the migration of wear debris into the acetabular bone region, it has been proposed to seal or cover the unused screw holes of an acetabular shell. For example, U.S. Pat. No. 5,925,077 shows an acetabular shell component having preinstalled plugs which plug screw holes in the acetabular shell. The plugs may be a plug that is press fit into a screw hole, or a disk shaped plug that is bonded over a screw hole, or a plug that is snap fit into a screw hole. If it is determined that fixation screws are needed for implantation of the acetabular shell, selected plugs may be removed using a removal instrument. U.S. Pat. No. 5,782,929 shows an acetabular shell having plugs which plug screw holes in the acetabular shell. The screw hole plugs are press fit and then sintered into the screw hole. The screw hole plugs can be removed by the surgeon to create an unplugged hole for receiving screws. U.S. Pat. No. 5,571,198 shows an acetabular shell having screw holes which are plugged with threaded screw hole plugs. The screw hole plugs can be removed by the surgeon to create an unplugged screw hole that can receive a screw. U.S. Pat. No. 5,370,702 shows an acetabular shell having removable portions that a surgeon may grasp by a post and pull out of the shell thereby creating a screw hole in the shell. U.S. Pat. Nos. 6,152,962, 6,120,546, 5,935,174, 5,876,456, 5,645,606, 5,609,648 and 4,955,325 all show other examples of the use of plugs to fill the screw holes in a acetabular shell. Thus, depending on the construction of the acetabular shell, the surgeon may either plug unused open screw holes or open plugged screw holes at the time of implantation of the acetabular shell.
The prosthetic systems of the above patents all provide various solutions to the problems associated with the migration of wear debris through screw holes of an acetabular shell and into the acetabular bone region. However, these prosthetic systems do have disadvantages. For instance, each of these acetabular shell implants requires the use of multiple plugs that must secured (at the manufacturing site or in the operating room) into holes in the acetabular shell by threading, sintering, press-fitting, or other similar technique. Acetabular shells that are supplied from the manufacturer with plugged screw holes can be quite expensive given the large number of manufacturing operations required to assemble the plugs into the screw holes of the acetabular shell and the costs associated with the manufacture of a large number of plugs. Acetabular shells that are supplied with plugged screw holes may also necessitate the use of special surgical instruments to unplug the screw holes in the operating room. Acetabular shells that are supplied from the manufacturer with unplugged screw holes require the hospital to purchase and store a large number of plugs (which typically will vary among manufacturer). Then, the surgeon must assemble the plugs into the screw holes during the implantation procedure. This usually requires the use of special instruments to insert the plugs into the screw holes.
Also, these known acetabular shells may not provide for maximum surface area for potential tissue ingrowth on the outer surface of the acetabular shell as the screw hole plugs may be positioned below the outer surface of the acetabular shell. When screw hole plugs are positioned below the outer surface of the acetabular shell, tissue ingrowth is limited to the outer surface of acetabular shell surrounding the screw hole plugs. Furthermore, these known acetabular shells may only include a limited number of screw holes, given the increased manufacturing costs associated with manufacturing additional plugs and inserting the plugs into a large number of screw holes. This limits the surgeon""s choices for bone screw location such that the quality and number of screws with excellent bone purchase may be less than is deemed desirable by the surgeon. Furthermore, the location of the few screw holes in these known acetabular shells may be incongruous with the location of available bone stock.
Therefore, there is a continuing need for an improved acetabular shell that allows for screw fixation of the acetabular shell to bone at multiple locations and that limits migration of wear debris out of the screw holes of the acetabular shell and into the acetabular bone region. In particular, there is a need for an acetabular shell that provides a large selection of available locations for screw fixation, that retains maximum outer surface area for bone tissue ingrowth, and that can also limit migration of wear debris out of the screw holes of the acetabular shell and into the acetabular bone region.
The foregoing needs are met by an outer shell for a socket of a joint prosthesis in accordance with the invention. The outer shell includes an outer wall defining an outer surface of the outer shell and an inner wall defining an inner surface of the outer shell. The outer wall has pores of a size sufficient to permit bone tissue to grow into the outer wall and the inner surface of the outer shell is suitable for engaging an outer surface of the socket. Optionally, the inner wall is joined to the outer wall thereby forming an interfacial boundary between the inner wall and the outer wall. The inner wall has a plurality of access channels suitable for seating a bone screw. There is no limit on the number of access channels and therefore, the shell provides a large selection of available locations for screw fixation to available bone. In one embodiment, the outer shell is an acetabular shell for the socket of a hip joint prosthesis.
At least a portion of the access channels of the outer shell are closed off by an associated integral section of the outer shell such that the portion of access channels do not extend through the outer surface of the outer shell. Each associated integral section includes at least a section of the outer wall. At least one of the associated integral sections may also include a section of the inner wall. At least one of the associated integral sections may also consist of the entire cross-sectional width of the outer wall. In other words, the particular section of the outer shell that closes off each access channel may comprise: (i) a section of the outer wall having less than the full cross-sectional width of the outer wall; (ii) a section of the outer wall having the full cross-sectional width of the outer wall; or (iii) a section of the outer wall having the full cross-sectional width of the outer wall and at least a cross-section of the inner wall. Each associated integral section is suitable for penetration with a self tapping bone screw or with a drill bit (or similar puncturing instrument) such that a surgeon may insert and seat bone screws in the opened access channel.
The outer wall comprises an integral unitary structure such that each closed off access channel is closed off by at least a section of the outer wall as described above. Therefore, there is no need for plugs or inserts to close off any access channels in the shell. The pores of the outer wall may be formed by interconnected metallic particles, such as particles formed from titanium, titanium alloys, cobalt alloys, stainless steel alloys, or tantalum alloys. Optionally, the outer surface of the outer shell has a plurality of depressions for mechanical interlocking with bone tissue, and the outer surface of the outer shell may also have a coating of a bone ingrowth promoting material such as hydroxyapatite. Because each closed off access channel is closed off by at least a section of the outer wall, the shell retains maximal surface area for bone tissue ingrowth on the outer surface of the shell. In other words, there are no plugs or inserts that hinder bone tissue ingrowth.
It is therefore an advantage of the present invention to provide an acetabular shell that limits migration of wear debris out of screw holes of the acetabular shell and into the acetabular bone region and that allows for screw fixation of the acetabular shell to bone at multiple locations.
It is another advantage of the present invention to provide an acetabular shell that limits migration of wear debris out of screw holes of the acetabular shell and that retains maximum outer surface area for bone tissue ingrowth.
It is a further advantage of the present invention to provide an acetabular shell that limits migration of wear debris out of screw holes of the acetabular shell and that does not require the use of multiple screw hole plugs or specialized plug removal equipment.
It is a yet another advantage of the present invention to provide an acetabular shell that allows for both screw fixation and press-fit implantation and that retains maximum outer surface area for bone tissue ingrowth.